A combination of an economic multi-band optical carrier generator and a novel optical signal to noise ratio (OSNR) enhancement circuit is proposed and demonstrated for radio over fiber (RoF) transport systems. Different from normal RoF transport systems which a central station (CS) needs multiple dedicate wavelength laser diodes (LDs) to support various base stations (BSs), the proposed scheme can employ a single LD to provide multiple optical carriers for various BSs. To verify this scheme, 8 coherent optical carriers are firstly generated using a single LD and a local oscillator (LO). Subsequently, their OSNR values are optimized by the developed OSNR enhancement circuit. An up to 15 dB OSNR enlargement in those optical carriers is experimentally achieved. To demonstrate the practice of the proposal, a pair of those optical carriers is employed to experimentally achieve frequency up-conversion process in a RoF transport system. Clear eye diagram and error free transmission reveal that with a proper carrier selector the proposed scheme can be employed to support multiple RoF transmissions. Furthermore, this proposal also presents a high possibility to achieve 60 GHz RoF transmission using a 10 GHz LO, a LD and a low frequency external modulator.
© 2011 OSA
Optical fiber with high capacity, low attenuation, light weight and electromagnetic noise interference (EMI) free characteristics are recently employed in broadband radio over fiber (RoF) transport systems [1, 2]. The applications of distributing radio frequency (RF) signals over wavelength division multiplexing (WDM) optical fiber transport systems bring a lot of benefits, including high-performance, wide-service range and easy management . Nevertheless, such RoF transport systems require multiple high frequency local oscillators (LOs) and mixers to up-convert baseband signal into RF domain as well as need various dedicated wavelength laser diodes (LDs) to support multiple RoF transmissions. The great amount of the required passive and active components will be a serious restriction in promoting RoF transport systems. To overcome this bottleneck, arbitrary multi-band optical carrier generator techniques are recently proposed to support RoF transmissions in optical WDM transport systems [3–7]. In those techniques, multiple equal-space coherent lightwaves can be generated by a LD, a LO and some external modulators. Each lightwave pair can then be employed to support a RoF transmission by externally modulating and transmitting with specific baseband data stream. Since the employed lightwaves are coherent with each other, a photonic detector (PD) can directly detect those two lightwaves and convert the contained data stream from baseband domain into RF domain [8, 9]. The central frequency of this RF signal will equal to the frequency difference between the detected lightwaves. In other words, employing a flexible multi-band coherent-lightwave generator in a central station (CS) is able to simultaneously support various high frequency RoF transmissions without any signal mixing process in electrical domain. No additional LO or mixer is required in the CS. The saved component cost can greatly reduce the CS construction expenditure. Nevertheless, the costly external modulators, such as phase modulator (PM) or Mach-Zehnder modulator (MZM), are needed in those multi-band coherent lightwave generation schemes - . In order to simplify such generator structure by taking off external modulators, an economic multi-band coherent-lightwave generator is developed in this proposal and the generated optical lightwaves are optimized by a novel optical signal to noise ratio (OSNR) enhancement circuit.
In contrast to external modulation schemes, directly modulating electrical signals with a LD is much more economic. However, a LD with limited laser resonance frequency (LRF) and chirping effect in nature is not suitable to be directly modulated with high frequency signals [10, 11]. To overcome this problem, sophisticate light-injection techniques and optoelectronic feedback techniques for examples have been developed to extend the LRF and to reduce the chirping effect . This proposal on the other side will not suppress the chirping effect but boost up this effect to generate multi-band optical carriers for various RoF transmissions. In literature, producing few-band optical carriers by directly modulating a sinusoidal RF source with a LD has been investigated and achieved , and M. Yoshino, et al. , have employed this characteristic to generate optical code-division multiplexing (OCDM) signals. However, a fact can be found from those schemes that the average OSNR value of the generated optical carriers is gradually reduced with the increased carrier number. Neglecting waveform distortion and the impact of crosstalk in signal itself, a limited OSNR value will significantly impact the receiver Q-factor value and as a result will degrade the relative bit error rate (BER) performance in a transport system. To put it another way, a reduced optical power variation between the signal level and the noise level will short the overall transmission distance. Therefore, a novel OSNR enhancement circuit is developed to optimize the proposed multi-band optical carrier generation technique for RoF transport systems.
In this proposal, multi-band coherent-lightwaves are generated by directly modulating a sinusoidal RF source with a commercial distributed feedback (DFB) laser and are optimized by the proposed OSNR enhancement circuit. With the assistance of this circuit, an up to 15 dB improvement in OSNR values is experimentally achieved. Open eye diagram and error free transmission demonstrate that this proposal not only can efficiently promote the input lightwaves but also can maintain their coherent characteristic at the same time.
2. Optical carrier generator configuration and design
Figure 1(a) illustrates the schematic diagram of the OSNR enhanced multi-band optical carrier generator. In this structure, multiple coherent lightwaves are firstly generated by directly modulating a sinusoidal RF signal with a DFB laser. Subsequently, they are optimized by the novel OSNR enhancement circuit. In an optical direct modulation system, the electric field of modulating a RF signal can be described using the following small signal approximation :14]. To extend this technique for RoF transport systems, a commercial DFB laser is employed in the proposed optical carrier generator and is experimentally modulated with a 10.8 GHz sinusoidal RF signal. The RF power level is varied from low to high to observe the relative DFB output spectra. The obtained OSNR values of the generated multi-band optical carriers without any optimization are shown in Fig. 1(b). The relative optical spectra of the OSNR observed points (a)-(d) at the Fig. 1(b) are presented in Fig. 2(a)-(d) respectively. It can be clearly observed from these figures that the number of the generated optical carrier is in direct proportion to the RF power level but the maximum OSNR value is in inverse proportion to the RF power level. As shown in the Fig. 2(d), multi-band optical carriers are successfully achieved in this experiment, but the variation of their OSNR values among wavelengths is seriously and the overall OSNR values are much smaller than that in the Fig. 2(a)–(c). The relationship between the OSNR value and the receiver Q-factor value can be displayed as :Eq. (3) below , a lower Q-factor value will cause a poor BER performance.
In this case, the application of the lightwave generation technique in RoF transport systems is limited by the poor BER performance. To overcome the drawback, a novel OSNR enhancement circuit is composed to re-shape the generated optical carriers with larger OSNR values and smaller OSNR variation among wavelengths. As presented inside the red line square in the Fig. 1(a), the OSNR enhancement circuit is composed by an optical circulator (OC), a delay interferometer (DI) and a reflective semiconductor optical amplifier (RSOA). The OC is employed to route the generated optical carriers into and out from the enhancement circuit. The DI acts as a comb filter and has free spectral range (FSR) equal to the modulated RF frequency. By carefully adjusting the DI working wavelength range or LD output wavelength, every generated optical carrier can be located in the pass-band of the DI and the valleys of the carriers will be located in the stop-band of the DI. As a result, the corresponding noise figures will be attenuated to lower values. Following with the DI, a RSOA is placed to boost up and reflect the inserted multi-band optical carriers, so the reflected optical carriers will be optimized one more time before be circuited out the enhancement circuit. Normally, a RSOA’s transfer function is slightly different for different inputted optical wavelengths, and its optical gain performance is generally in inverse proportion to the input power . In this case, each inserted optical carrier will have slightly different gains, and the noise figures in the 1549.5nm region will have a larger gain than that in the 1550.0nm region. After the carriers are filtered by the DI again, their OSNR values can be extent and the variation can be reduced. The optical spectrum diagrams of the generated multi-band optical carriers with and without OSNR enhancement are shown in Fig. 3(a) . It is clear that the OSNR variation among wavelengths is significantly optimized. To numerically evaluate the effect, the OSNR values of the central 11 optical carriers with and without enhancement are recorded in Fig. 3(b). Originally, the maximum OSNR value among these carriers is limited at around 22 dB and the variation among them is more than 6 dB. Once the generated optical carriers are optimized by the proposed circuit, each OSNR value is promoted to around 31 dB and the variation is reduced to around 2.5 dB. A maximum 15 dB OSNR improvement is experimentally achieved at around 1550 nm. It is expected that this optimization can cause a better receiver Q-factor value in a RoF transport system making the multi-band optical carrier generation technique more practice.
3. Experimental setup
In order to demonstrate the practicability of the proposal in RoF transport systems and to test whether the OSNR enhancement circuit can optimize the input optical carriers and maintain their coherent characteristic at the same time or not, an experiment was configured as shown in Fig. 4 . In this experiment, a 10.8 GHz sinusoidal signal was directly modulated with a DFB laser to produce multi-band optical carriers. Subsequently, these carriers were fed into the investigated OSNR enhancement circuit to extend their OSNR performance and then amplified by an EDFA. The optimized carriers, as shown in the insert (i) of the Fig. 4, were routed and selected by an OC and a 20 GHz fiber Bragg grating (FBG). Two of them were externally modulated with a 100 Mbps pseudo-random binary sequence (PRBS) by a MZM. Subsequently, these two modulated carriers, as shown in the insert (ii) of the Fig. 4, were detected by a PD and then analyzed by a digital communication analyzer (DCA). If the optimized carriers still maintain coherent characteristic, the modulated baseband signal can be detected and frequency-up-converted to 10.8 GHz range by the PD. So that, no more LO or mixer is required to up-convert the PRBS signal into RF range in the CS.
In the experiment, we had tried to demonstrate the lightwave coherent characteristic by selecting and employing two of the multi-band optical carriers to transmit RoF signal. Nevertheless, due to the absent of a suitable wavelength selector, the unwanted optical carriers were not entirely depressed; they still presented in the receiver end. Fortunately, this situation can be eliminated by utilizing a better optical wavelength selector and the employed FBG is still able to demonstrate the coherent characteristic of the optimized lightwaves. The measured BER curve is shown in Fig. 5 . The electrical spectrum diagram of the PD-detected 100Mbps/10.8GHz RF signal and the corresponding eye diagram are also presented in the insert (i) and insert (ii) of the figure, respectively. A clear pulse is observed at the frequency of 10.8 GHz which is exactly the same with the frequency difference between two selected optical carriers. This result demonstrates a success in optimizing comb style lightwaves in the same coherent characteristic. Since all of the carriers are coherent with each other, employing a 10.8 GHz LO in our architecture is possible to achieve a 2 times (21.6 GHz), 3 times (32.4 GHz), 4 times (43.2 GHz) or even an up to 7 times (75.6) GHz) frequency up-conversion process in optical domain. To pout it another way, the OSNR enhanced multi-band optical carrier generator not only can be employed to support multiple RoF transmissions but also can be utilized to achieve 60 GHz RoF transmission using a low frequency LO and modulator. The practice of the proposed scheme can also be found from the measured BER and eye diagrams. An error free transmission (BER >10−9) is experimentally achieved and a clear and open eye diagram is also obtained in the received end. Although the receiver sensitivity will be degraded by fiber dispersion induced by a long distance transmission, sophisticated and expensive high-bandwidth RF devices as well as high frequency LOs and optical modulators are not used in the architecture; it presents a feasible way with more economic advantages to support RoF transport systems. All of the experimental results become great evidences to demonstrate the practice of the proposed multi-band optical carrier generation technique.
An OSNR enhanced multi-band optical carrier generator is proposed and experimentally demonstrated for RoF transport systems. In this proposal, coherent multi-band optical lightwaves are generated using a single LO and a DFB. Subsequently, the obtained lightwaves are optimized by the developed OSNR enhancement circuit. To verify the practice of the proposal, up to 15 dB improvements in OSNR values are experimentally achieved for the generated optical carriers and an error free transmission and a clear eye diagram are also achieved. These results prove that the OSNR enhancement circuit not only can optimize the input optical carriers with better flatness and OSNR values but also can maintain their coherent characteristic to support various RoF transmissions. From the experimental results, we can expect that the developed multi-band optical carrier generation technique is able to achieve an arbitrary multifold (1~7 times) frequency up-conversion process in optical domain. This means that the proposed scheme can employ a 10 GHz LO, a DFB and a low frequency external modulator to achieve 60 GHz RoF transmission.
The authors would like to thank the financial support from the National Science Council of the Republic of China under Grant NSC 100-2221-E-027-067-MY3 and 100-2218-E-415-001.
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